1. Field of the Invention
The invention relates to an electrostatically operating electroacoustic transducer functioning as a sound receiver and employed in a microphone capsule. Independent of the way they operate with regard to physical principles, such transducers have a diaphragm exposed to a sound field and directly excited by the sound field so as to perform vibrations.
2. Description of the Related Art
The subject matter of the invention is thus an electrostatic microphone. The electrodes of the electrostatic transducer are in the form of an elastic, taut diaphragm and a rigid electrode which is usually referred to simply as electrode. Both form a capacitor whose capacitance changes as a result of the pressure fluctuations of the sound field. Since an electric field is generated between the electrodes of the electrostatic transducer, it is possible to transform the capacitance changes of the transducer by means of an amplifier arranged downstream into electric voltage changes.
Electrostatic capsules can be divided into two groups based on the way the electric field between its electrodes is applied:
1. Electrostatic capsules in which the charges are applied by means of an externally supplied voltagexe2x80x94capacitor capsules;
2. Electrostatic capsules in which the charges are xe2x80x9cfrozenxe2x80x9d on the electrode or diaphragm so that in this way an externally supplied voltage becomes obsolete electret capsules.
In addition to various advantages relative to other sound receiving types, the two aforementioned types of electrostatic sound receivers have a major disadvantage: they are sensitive to the humidity in the air. Since, considered electrically, the above described capacitor is a high-resistivity device, it is absolutely mandatory for a successful electroacoustic transformation that the first stage of the amplifier arranged downstream is also of high resistivity. It is apparent that increased humidity in the air in the case of high-resistivity electric devices presents a great risk with respect to their reliability. A 100% protection of the amplifier against negative effects of the humidity in the air can be obtained by a consequent application of the known lacquering measures. In this connection, sealing by means of different types of lacquer, varnish, or enamel successfully prevents that the humidity of the air can negatively affect the electric properties of the amplifier. However, in the case of a capsule, the protection against humidity is much more complicated. The only direct insulation path between the electrodes of the capacitor is the spacer ring. An increased humidity results in condensation which has a negative effect on the quality of the insulation path in the area of the spacer ring and is recognizable as an unpleasant crackling sound during operation of the microphone.
According to the present solutions of the prior art, this problem is solved in that the insulation path between electrode and diaphragm is to be made as large as possible. However, this leads to larger and more expensive capsules with still only a small or unsatisfactory improvement of the resistance to humidity.
It is an object of the present invention to provide a better protection than in the past against the risk of short-circuiting between the electrodes of a microphone of the aforementioned kind in the presence of high ambient humidity without this increasing significantly the size of the microphone capsule or its manufacturing costs.
In accordance with the present invention, this is achieved in that on the rigid electrode, preferably on the side facing away from the diaphragm, at least one electric resistor is arranged, preferably by gluing, which is connected to a power supply during operation of the microphone.
With this measure the rigid electrode and thus its edge area and therefore the sensitive area where the diaphragm is secured is heated by a few fractions of degrees to a temperature above the ambient temperature; this is already sufficient to prevent condensation reliably. Since on the backside of the electrode there is always a hollow space provided, which is mandatory for reasons of acoustic tuning, the dimensions of the capsule are not changed.
Supplying power does not present a problem and can be realized by separate lines through which hardly any power must be transmitted and which therefore can be configured to be thin. As is known in the art, microphones, which operate on the basis of electrostatic principles, require a power supply for supplying energy to the amplifier downstream of the capsule which, in turn, can be a separate battery supply or in the form of a so-called phantom power supply (audio mixer). This phantom power supply can also be used for operating the resistor.